Heat pumps have been used for many years in the heating and cooling of buildings; their popularity has substantially increased in recent times because of the soaring costs of energy used for heating and cooling. Heat pumps become more and more attractive for the function of heating and cooling of buildings because of their operating efficiency; i.e., their cost effectiveness. However, heat pumps do have some problems; one of these is connected with the fact that in many systems the refrigerant in the line may, during times that the system is at rest, settle in the crankcase of the compressor. This is because, in the system "OFF" condition, the refrigerant in the reverse cycle heat pump will tend to condense at the location which has the lowest temperature in the system. The "coldest" location typically is in the outdoor unit (where the compressor is usually located) when a system is in the heating mode, because the outdoors is generally much cooler than the indoors for this case. Thus, the refrigerant may settle, i.e., condense in the crankcase of the compressor; the refrigerant will continue condensing at such coldest location until a point of equilibrium is reached, i.e., an equilibrium of liquid and gaseous refrigerant at the vapor pressure corresponding to the temperature at such coldest location. It has been recognized heretofore that it is important not to start up the compressor when the refrigerant has settled in the compressor crankcase as it is known that the refrigerant in the crankcase will tend to mix with the compressor lubricating oil therein. It is likely that this mixture is present at equilibrium because the mixture causes a reduction in the total volume of liquid as compared with a system containing separate pools of oil and refrigerant, thus enabling more refrigerant to condense at the same equilibrium vapor pressure. Thereafter, when the compressor is started, if there is refrigerant in the crankcase oil, then such refrigerant will tend to boil due to the low pressure on the suction side of the compressor (where the crankcase is located) and when this happens the refrigerant will agitate the oil causing the oil to foam; this foam then is apt to be carried into the intake of the compressor and thereafter be pumped out by the compressor into the refrigerant lines. When this happens, the oil may be pumped out of the crankcase, thus causing the compressor to run without lubricant until the oil migrates back having travelled throughout the complete refrigeration system; i.e., back through the refrigerant tubes and into the crankcase. Such running without lubrication may cause severe wear and overheating of the compressor, thus shortening the life of the compressor and causing expense, inconvenience and discomfort. Another related problem is that the oil refrigerant foam mixture is not as compressible as refrigerant vapor; this can cause "slugging" and eventual damage to the valves of the compressor.
All of the foregoing has heretofore been recognized and various prior art techniques have been proposed for dealing with the problem. Thus, at this time, many heat pump compressors have some means for heating the crankcase of the compressor so that the crankcase will not be the lowest temperature point in the heat pump system; thus preventing the refrigerant from condensing in the crankcase and thus preventing the above-described damages to the compressor. Thus, a frequent practice has been, in connection with the installation of a new heat pump system, to refrain from starting up the compressor for a period of time allowing the crankcase heating means to vaporize any accumulated refrigerant in the crankcase. However, frequently in practice (either through carelessness or ignorance) the heat pump installer will energize or turn on the compressor immediately; i.e., without waiting for the warming up interval, and hence cause damage to the compressor. Also, a crankcase heater failure will cause every compressor start with potential to dying. Also, an extended heater power loss could cause foaming.
It is an object of our invention to provide a new and effective system for detecting compressor crankcase low differential temperatures and for inhibiting the operation of the heat pump compressor until such time as the crankcase temperature increases above the outdoor air temperature to a safe level.